1. Field of Invention
The invention relates to an ATM (Asynchronous Transfer Mode) communications system and method. More specifically, the invention relates to a communications system and method using the ATM. The UTOPIA interface between the ATM layer and the PHY layer in a network communications protocol stack is improved.
2. Related Art
A protocol is like a language. Through a specific communication protocol, information can be communicated between two parties. The protocol is processed between layers and provides services from the low layer to the high layer. In order to transmit data between the layers successfully, both of them have to use the same protocol. Therefore, the protocol plays an important role in digital communications. Without a common protocol, users cannot communicate with each other at all.
In communication, the most famous model of protocol stack is the OSIRM (Open System Interconnect Reference Model) with seven layers. The ATM basic reference model is based upon the physical layer and the data link of the OSIRM. As shown in FIG. 1, the ATM basic reference model includes an ATM physical layer (PHY) 101, an ATM layer (ATM) 102, an ATM adaptation layer (AAL) 103, and other higher layers 104. The UTOPIA (Universal Test & Operation PHY Interface for ATM) 105 is a standard communications interface mainly used between the ATM 102 and the PHY 101.
In the ATM basic reference model, the PHY 101 is equivalent to the physical layer of the OSIRM, providing a transmission channel for data cells in the ATM layer 102. The data cells transmitted from the ATM layer 102 is added with an overhead to form a continuous bit stream. After receiving a continuous bit stream transmitted from a medium, effective data cells are extracted and sent to the ATM layer 102. The ATM layer 102 companied with the AAL layer 103, equivalent to the data link in the OSIRM, is responsible for the network link establishment and data cell transmissions. The AAL layer 103 is in charge of adapting high layer protocol packet to data cells. Other higher layers 104 receive and pack user data and send them to the AAL 103.
The processing unit of the ATM is a data cell. In the ATM standard, the data cell has a fixed length of 53 bytes, within which a header occupies 5 bytes and a payload uses the rest 48 bytes. In the data cell, each bit is transmitted in the transmission path in a continuous stream. Each data cell belongs to an actual transmission path.
The standard UTOPIA interface 105 is the one mainly used between the ATM layer 102 and the PHY 101. The ATM layer 102 inquires the PHY 101 through polling to obtain response signals of whether data should be transmitted or received before data are actually sent or received.
The working models of the UTOPIA include Level 1 and Level 2. The working model of UTOPIA Level 1 is a 1ATM-1PHY architecture, as shown in FIG. 1. There are four working models for UTOPIA Level 2: (1) 1 ATM-1 PHY, (2) 1 ATM-Multi PHY, (3) Multi ATM-1 PHY, and (4) Multi ATM-Multi PHY. However, most chips with the UTOPIA Level 2 interface only support 1ATM-Multi PHY. Therefore, we will take the UTOPIA Level 2 (2) 1 ATM-Multi PHY as an example to explain the standard UTOPIA interface technology.
FIG. 2 shows signals transmitted between the 1ATM-Multi PHY in the standard UTOPIA Level 2 communications interface. In the transmitting interface, the ATM layer 102 provides TxClk, TxAddr, TxEnb, TxData, and TxSOC signals to the multi PHY 101. The Multi PHY 101 sends a TxClav signal to the ATM layer 102. The TxClk signal is a synchronized clock from the ATM layer 102 that synchronizes the Multi PHY 101. The TxAddr signal contains 5-bit long positioning address sent from the ATM layer 102 for polling the PHY 101 or assigning transmissions on a selected PHY 101 when acting along with the TxEnb signal. The TxEnb signal is a signal sent out from the ATM layer 102 to enable the data cell transmission for the transmitting interface. As said before, it can act along with the TxAddr signal to assign transmissions on a selected PHY 101. The TxData signal is a channel through which the ATM layer 102 transmits data to the PHY 101. The TxSOC signal is an indication signal of the beginning of the data cell. The TxClav is a signal that is used by a PHY 101 to respond to the polling from the ATM layer 102.
With reference to FIG. 3A, the timing diagram of UTOPIA transmitting interface, the ATM layer 102 inquires each PHY 101 whether data can be transmitted before the data are really transmitted by polling. At this moment, the ATM layer 102 uses the TxAddr signal to appoint a PHY 101 to inquire. The inquired PHY then uses the TxClav signal to respond its availability to the ATM layer 102 at the next clock. Therefore, the ATM layer 101 can inquire one PHY every two clocks. Through the PHY availability states collected by polling, the ATM layer 101 determines the transmissions of data cells (using TxEnb, TxAddr, TxSOC, and TxData). The UTOPIA thereby completes data cell transmissions and flow controls.
With further reference to FIG. 2, in the receiving interface, the ATM layer 102 provides RxClk, RxAddr, and RxEnb receiving signals to the Multi PHY 101. The Multi PHY 101 provides RxData, RxSOC, and RxClav receiving signals to the ATM layer 102. The RxClk signal is a synchronized clock from the ATM layer 102 that synchronizes the Multi PHY 101. The RxAddr signal contains 5-bit long positioning address sent from the ATM layer 102 for polling the PHY 101 or assigning transmissions on a selected PHY 101 when acting along with the RxEnb signal. The RxEnb signal is a signal sent out from the ATM layer 102 to enable the cell transmission for the receiving interface. As said before, it can act along with the RxAddr signal to assign transmissions to a selected PHY 101. The RxData signal is a channel through which the ATM layer 102 transmits data to the PHY 101. The RxSOC signal is an indication signal of the beginning of the data cell. The RxClav signal is a signal that used by the PHY 101 to respond to the polling from the ATM layer 102.
With reference to FIG. 3B, the timing diagram of UTOPIA receiving interface, the ATM layer 102 inquires each PHY 101 whether data is available for transferring before the data are indeed transferred by polling. The inquired PHY then responds with the RxClav signal. The ATM layer 101 can inquire one PHY every two clocks. Through the PHY availability states collected by polling, the ATM layer 101 determines the transmissions of data cells (using RxEnb, RxAddr, RxSOC, and RxData). The UTOPIA thereby completes data cell receptions and flow controls.
From FIGS. 3A and 3B, one knows that the communications between the ATM layer 102 and the PHY 101 have to satisfy the timing defined by the standard UTOPIA interface 105 in order to correctly transmit/respond information. If the timing of the UTOPIA polling has some errors, the ATM layer 102 will receive incorrect availability state response from the PHY 101. This will result in failure of the flow control mechanism and cause errors in transmitting/receiving data cells. If there are errors in the timing of the UTOPIA data cell transmit interface/receive interface, it may induce errors into the cell content. That is, in effect, the incorrect timing in polling and/or cell transmit interface/receive interface can result in fetal errors to the UTOPIA interface.
Under normal conditions, the ATM device 13 and the PHY device 15 are directly connected. The UTOPIA interface can assure normal operation of data transmissions between the ATM device 13 and the PHY device 15. However, due to high port density and high-speed switching demands for modem communication systems, the communication system structure becomes somewhat more complicated. Under some conditions, system designers hope that the UTOPIA ATM device 13 can connect to the UTOPIA PHY device 15 through a transmission medium 10. A consequence of this is that the UTOPIA work timing between the ATM device 13 and the PHY device 15 cannot be compliant with the standard UTOPIA timing. If no other mechanism is used to conquer this problem, the communication system cannot use such a structure to reach expected effects.
To solve the timing problem, the widely used and most direct method is to introduce two buffers 11, 12 in the upper and lower sides of the transmission medium 10 (see FIG. 4). In this structure, the upper buffer 11 must support the standard UTOPIA PHY interface to interact with the UTOPIA ATM device 13 in the same way as the lower buffer 12 supports standard UTOPIA ATM interface to interact with the UTOPIA PHY device 15. In the Tx direction (cell transferred from ATM device to the PHY device), the upper buffer 11 receives and buffers cells that transmitted from the ATM device 13. The cells are then transferred to the lower buffer 12 through the transmission medium 10. Afterwards, the lower buffer 12 regenerates the cells in its standard UTOPIA ATM interface and transfers the cells to the UTOPIA PHY device 15. Similarly, in the Rx direction (cell transferred from PHY device to the ATM device), the cells transmitted from the UTOPIA PHY device 15 will be received and buffered by the lower buffer 12, transferred through the transmission medium 10, regenerated by the upper buffer 11 in its standard UTOPIA PHY interface, and finally arrive at the UTOPIA ATM device 13. In this way, the timing problem can be solved.
However, this method has extra steps in signal transmissions between the ATM device 13 and the PHY device 15, and results in the introduction of unnecessary transmission delay. Not only is the system structure more complicated, the buffers' add-in also increases the cost. Therefore, this invention proposes a communication system/method that uses an improved UTOPIA interface between the ATM layer and PHY layer, so that no extra devices are needed in a high-speed transmissions structure described in the preceding paragraph. In comparison with the prior art, the invention indeed achieves the goals for a simplified structure, higher speed and lower cost.
An objective of the invention is to propose an improved UTOPIA interface. We call it a Pseudo-UTOPIA interface. The Pseudo-UTOPIA interface accurately adjusts the standard UTOPIA interface timing mechanism so that the ATM device that supports the disclosed interface can easily connect to a standard UTOPIA PHY device through another high-speed transmission medium. The ATM device and the standard UTOPIA PHY device can achieve optimal transmission efficiency (minimal transmission delay) through a high-speed transmission medium in an economical way (without introducing extra buffers).